Mullite-containing coatings for inorganic fibers and ceramic matrix composites

ABSTRACT

A ceramic matrix composite article comprised of inorganic fibers having a mullite-containing coating disposed within a matrix phase. The invention also provides a method for mating such an article, as well as for preparing a fiber having a mullite-containing coating. The mullite-containing coating on inorganic fibers within a matrix acts as a debonding coating, and the ceramic matrix composite article exhibits high strength and fracture toughness, even at elevated temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application60/000,688, filed Jun. 21, 1995 and PCT International ApplicationPCT/US96/10625, filed Jun. 19, 1996, wherein the United States was adesignated country.

FIELD OF THE INVENTION

The invention generally relates to composite articles. In particular,the invention relates to inorganic fibers having a mullite-containingcoating and to ceramic matrix composite articles reinforced with saidfibers.

BACKGROUND OF THE INVENTION

Fiber-reinforced ceramic matrix composites comprising glass-ceramicmatrices are known in the art. Fiber-reinforced ceramic matrixcomposites are useful as structural elements in high temperatureenvironments such as heat engines. For these and other applications, thematerials to be employed must exhibit good strength and toughness atambient as well as elevated temperatures.

An important problem which has been identified in silicon carbide fiberreinforced ceramic matrix composites, particularly after exposure totemperatures above about 800° C. in an oxidizing environment, is thatmicrocracks can form causing embrittlement. Instead of exhibitingincreased toughness and strength after exposure to high temperatures,the materials become brittle and are subject to catastrophic breakage,rather than more gradual failure as is typical of the original material.These physical problems can be attributed, in-part, to the effect of theinterface between the silicon carbide fibers and the ceramic matrixcomposite.

Physical testing of ceramic matrix composites, embrittled during orsubsequent to high temperature exposure, shows decreases in fracturetoughness through changes in the fracture properties of the material,leading to a degradation of the material. Thus, the predominant fracturemode changes from one characterized by fiber pullout from the matrix toone wherein woody fracture, or ultimately, brittle fracture occurs.Woody fracture surfaces display some crack propagation parallel to thestress axis, indicating localized shear failure without fibrous pullout,while brittle fracture surfaces display merely planar fracture surfacesas the composite exhibits no plastic deformation.

The onset of brittle fracture behavior in these composites typicallyoccurs in conjunction with significant reductions in fracture toughness.One indicator of this reduced toughness is a drop in the extent ofstrain of sample elongation observed above the so-called microcrackstress point of the material. Among the factors believed to influencefracture toughness are fiber debonding and fiber pullout behavior,including the degree of frictional resistance to fiber pullout from thematrix, as well as crack deflection occurring in the matrix and at thefiber-matrix interface. Modifications to the matrix or fiberreinforcement can significantly aid in the development of compositesexhibiting good high temperature fracture toughness and strength.

It is known to provide coatings on reinforcement fibers to beincorporated in composite materials to modify the behavior of thematerials therein. For example, boron nitride coatings have been appliedto silicon carbide fibers or other fibers that are subsequentlyincorporated in ceramic matrix materials such as SiO₂, ZrO₂, mullite andcordierite (see e.g., U.S. Pat. No. 4,642,271 (Rice)).

It is established that the interface between fibers and the matrix iscritical to the mechanical properties of brittle-matrix composites. Inparticular, the debonding and frictional characteristics of theinterface control the mode of fracture (multiple cracking vs. singlecrack), and mechanical properties such as fracture toughness. Desiredinterfacial properties are usually achieved by the incorporation of acoating between the fiber and matrix.

For example, Beall et al., European Patent Application PublicationNumber 366234 A1, disclose ceramic matrix composite articles comprisinga ceramic, glass-ceramic or glass matrix and a fiber reinforcement phasedisposed within the matrix. The fiber reinforcement phase consists ofamorphous or crystalline inorganic fibers, wherein there is provided, onor in close proximity to the surfaces of the inorganic fibers, a layerof sheet silicate crystals. The layer of sheet silicate crystals areused to improve fiber pullout behavior and to improve toughnessretention at elevated temperatures.

At present, however, there are only a few other successful coatingmaterials, most notably, carbon, although some success has been reportedwith metallic and porous coatings. In most of the composite systems thathave been studied to date, exposure of the coating to high temperaturesin air seriously degrades its properties. For example, in the case oflithium aluminum silicate matrix reinforced with carbon-coated siliconcarbide fibers, heat treatment in air leads to a strong SiO₂ interface,and the material loses its quasi-brittle mechanical properties. There istherefore great interest in developing alternative coatings for fibersin brittle-matrix composites.

Oxides are a class of materials which have intrinsic high temperaturestability in air. A particular interest has been to look at usingoxidation resistant materials as potential fiber coatings in ceramicmatrix composites for high temperature/high stress applications. Animportant consideration in choosing an interfacial material is itsability to form uniform coatings on the fibers in question.

Clearly, coating materials having excellent film-forming capability, andwhich can be coated successfully onto fibers such as SiC andborosilicate glass fibers, are needed. Such materials need to providedebonding coatings on the surfaces of fibers used in ceramic matrixcomposites, wherein such coatings remain stable at elevatedtemperatures. Moreover, such ceramic matrix composites need to have highstrength and fracture toughness, even at elevated temperatures. As aresult, it is an object of the present invention to provide coatings forfibers and ceramic matrix composites that overcome the problems anddeficiencies of the prior art. Other objects and advantages of thepresent invention will become apparent to those skilled in the art uponreference to the attached drawings and to the detailed description ofthe invention which hereinafter follows.

SUMMARY OF THE INVENTION

The invention provides for an improved ceramic matrix composite articlecomprising:

(a) a matrix phase comprised of a ceramic material selected from thegroup consisting of crystalline ceramics, glass ceramics, glasses, andcombinations thereof; and

(b) a fiber reinforcement phase comprised of a plurality of amorphous orcrystalline inorganic fibers disposed within the matrix phase,

wherein the improvement comprises the inorganic fibers having amullite-containing coating on the surface of said inorganic fibers andwherein the inorganic fibers are not comprised of a mullite-precursor.

The invention also provides for a method of coating an inorganic fiberwith a mullite-containing coating, comprising the steps of:

(a) modifying a smectite clay by ion-exchange in solution to providesufficient amounts of aluminum ions in the clay;

(b) adding a pillared smectite clay to the solution to form asuspension;

(c) drawing an inorganic fiber through the suspension of step (b) anddrying the fiber thereafter;

(d) repeating step (c) a plurality of times until the desired amount ofcoating is deposited on the surface of the fiber; and

(f) heating the coated fiber to a temperature sufficient to convert theclay coating to a coating containing mullite. Optionally, excess saltsmay be removed after step (b) and before step (c).

The above procedure can be modified slightly to provide for making aceramic matrix composite article. In this modified process, prior tostep (f), the following step can be incorporated:

(e) combining a plurality of coated fibers with a matrix phase comprisedof a ceramic material selected from the group consisting of crystallineceramics, glass-ceramics, glasses and combinations thereof, to form aceramic matrix composite such that the plurality of coated fibers aredisposed within the matrix phase.

The invention also provides an amorphous or crystalline inorganic fiberhaving a mullite-containing coating wherein the inorganic fiber is not amullite precursor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) shows room temperature x-ray diffraction data for bentoniteand FIG. 1(b) shows comparative room temperature x-ray diffraction datafor Al-bentonite clays, both as a function of thermal treatment: (a) and(f) are at room temperature; (b) and (g) are after heating to 500° C.;(c) and (h) are after heating to 800° C.; (d) and (i) are after heatingto 1000° C.; and (e) and (j) are after heating to 1200° C.

FIG. 2(a) is a SEM micrograph of a SiC fiber with a bentonite claycoating.

FIG. 2(b) is an optical micrograph of a cross-section of the glass/SiCcomposite.

FIG. 2(c) is a SEM micrograph of the interfacial coating between theglass and SiC fiber (single dip).

FIGS. 3(a) and 3(b) are SEM micrographs of the residual displacement ofa fiber in a ceramic matrix composite after indentation of the fiber inthe matrix (FIG. 3(a) uncoated and FIG. 3(b) coated).

FIG. 4 shows typical results of four-point bend tests on four types ofspecimens: (1) glass matrix material; (2) glass matrix with uncoatedfibers; (3) Al-bentonite (i.e., mullite) coated fibers in glass matrixtested in air; and (4) Al-bentonite coated fibers in glass matrix testedin water.

FIG. 5 is an SEM showing multiple matrix cracks in an Al-bentonitecoated sample after testing in water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides for novel ceramic matrix composites, comprising amatrix phase and a fiber reinforcement phase wherein the fibers have amullite-containing coating on their surfaces. Novel inorganic fibershaving a mullite-containing coating are also provided.

The invention further provides for a novel process for thelow-temperature formation of mullite-containing coatings, from pillaredsmectite clay precursors, on fibers, for use in ceramic matrixcomposites (CMC's). Smectite clay precursors display excellentfilm-forming capability and can be uniformly coated onto inorganicfibers. Mechanical tests on composites of such coated fibers in a glassmatrix demonstrate the fibers are successful as debondable coatings. Inparticular, alumina-pillared bentonite can be converted substantially tomullite at the unusually low temperature of about 800° C.

One embodiment of the invention involves a ceramic matrix compositearticle. As is known in the art, composites typically comprise a matrixphase and a fiber reinforcement phase. The fiber reinforcement phase istypically combined with, and hence disposed within, the matrix phase,the two phases then being heated to form a composite article.

The improvement lies in the fact that the inorganic fibers in the fiberreinforcement phase have a mullite-containing coating on their surface,thus forming an interfacial layer between the fibers and the matrix. Incomposites, these coated fibers show debonding characteristics, as wellas evidence of being tough materials, i.e., multiple matrix cracking andnon-linear stress-strain response prior to peak stress. Additionally,since mullite is an oxide, stability at elevated temperatures isachieved.

As used herein, "mullite-containing coating" means a coating thatcomprises between about 1-100 wt. % mullite (i.e., an orthorhombicsilicate of aluminum, Al₆ Si₂ O₁₃ or 3Al₂ O₃.2SiO₂), although otherphases may be present. Preferably, the coating is at least about 40 wt.% mullite.

The matrix phase can comprise a ceramic material selected fromcrystalline ceramics, glass-ceramics, glasses and combinations thereof.This can include borosilicate glasses, aluminosilicate glasses, lithiumaluminosilicate glasses, and alkaline earth aluminosilicate glasses,silicon carbide, boron nitride, silicon oxynitride and silicon nitride.

The fiber reinforcement phase comprises amorphous or crystallineinorganic fibers having a mullite-containing coating on their surfaces.Useful inorganic fibers include fibers having one of the followingcompositions: silicon carbide, boron nitride, silicon oxycarbide,carbon, alumina, boron carbide, zircon, spinel, silicon nitride, siliconoxynitride, titanium carbide, and titanium diboride.

A proportion of at least about 1% fiber reinforcement by volume ispreferred to be included in the ceramic matrix composite of theinvention herein. Most preferably, the percentage of fibers by volumeshould be in the range of about 30 to 80%. The fibers may vary in sizeand shape, since this aspect is not considered critical to theinvention.

Layered aluminosilicate oxides have been found to be good film-formerswhich can be subsequently thermally, and in some cases, chemicallyaltered to give new coatings with interesting properties, such as amullite-containing coating. Pillared smectite clays are used asprecursor materials, which can be coated onto inorganic fibers. Beinginexpensive and excellent film-formers, these clays show promise asfiber coatings for brittle-matrix composites. Being oxides, they provideenhanced stability at elevated temperatures.

An important feature of these clay-based materials is the ease withwhich thin films can be formed. FIG. 2(a) shows a SiC fiber which hasbeen coated with a bentonite clay. The coating is seen to be uniformafter three dips and calcination at 500° C. The clay suspension in waterhas good film-forming properties as compared to analogous sol-gelderived suspensions where excessive cracking occurs upon drying. DrawingSiC fibers through an ion exchanged-bentonite suspension readily coatsthe fiber to uniform thicknesses of 50-250 nm, and the process can berepeated to build-up coatings of different thicknesses.

FIG. 2(b) shows a transverse section of a composite specimen,hot-pressed at 900° C. FIG. 2(c) shows the thin interfacial coating(approximately 30 nm). The presence of the coating was confirmed byenergy dispersive x-ray analysis (EDX), which showed aluminum at theedge of the fiber.

Montmorillonite (ideally, [Na₀.33.xH₂ O]^(1/3+) [(Al₁.67 Mg₀.33)Si₄ O₁₀(OH)₂ ]^(1/3-)) is a preferred smectite clay which consists of hydratedions charge compensating for and sandwiched between partiallysubstituted 1:2::Al:Si aluminosilicate layers. Bentonite is a naturallyoccurring montmorillonite-related mineral. FIGS. 1(a) and 1(b) comparethe structural evolution of bentonite and [Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺-exchanged bentonite as a function of thermal treatment, see Example 1.

As provided for in the process embodiment of the invention, clay-basedprecursors are used to form oxide coatings on inorganic fibers. Thealumina-pillared bentonite clay which is coated on the inorganic fibercan be transformed to mullite at temperatures as low as about 800° C.Thus, as noted before, the invention also provides for inorganic fibershaving a mullite-containing coating on their surfaces.

Composites can be formed by combining the mullite-containing coatedfibers with a matrix phase. Composites of fibers coated with amullite-containing coating in a borosilicate glass show considerabledebonding in indentation tests, and all the signatures of a toughmaterial: multiple matrix cracking and a nonlinear stress-strainresponse prior to peak stress. Since the coefficient of thermalexpansion of mullite is well-matched to that of SiC, a preferred matrixand inorganic fiber phase material is silicon carbide.

Hot-pressed composites of clay-coated SiC fibers with borosilicate glasshave been fabricated and mechanical tests using single-fiber indentationand four-point bending of composite beams have been used to study thepotential of such coatings as debonding interfaces. Indentationexperiments conducted on composites with coated and uncoated fibersunder loads in the range of 5-30 N showed residual displacement afterindentation of an order of magnitude greater for the coated fiberscompared to uncoated fibers (see Example 2). FIGS. 3(a) and 3(b) showSEMI micrographs of coated (3(b)) and uncoated (3(a)) fibers afterindentation.

Another embodiment of this invention involves a method for making aceramic matrix composite article. The first step of the method comprisesmodifying a smectite clay by ion-exchange to provide sufficient amountsof aluminum ions in the clay. This can be accomplished by forming asolution containing a soluble aluminum ion, preferably apolyoxo-aluminum Keggin ion, ([Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺). Othercations or cationic clusters are contemplated, e.g., Al³⁺, Ga based orGa/Al based clusters. Thereafter, a pillared smectite clay, e.g.,montmorillonite/bentonite, is added to the solution to form asuspension. The resulting ([Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺)-exchangedbentonite will be referred to as Al-bentonite, for purposes of thisinvention.

The next step in the method involves drawing an inorganic fiber throughthe suspension. By doing so, a coating of the suspension is deposited onthe surface of the inorganic fiber. After coating, the fiber should bedried to solidify the coating. This is generally accomplished by heatingthe fiber, typically to a temperature ranging from about 150° C. toabout 250° C. After drying, the inorganic fiber can be drawn through thesuspension a number of additional times until the desired thickness ofcoating is achieved, making sure to dry the fiber as described aboveafter each successive coating. The concentration of the smectite clay inthe suspension may vary depending on the desired coating thickness onthe fiber. Typical coating thickness will range from about 50 nm toabout 250 nm.

The next step in the method involves combining a plurality of coatedinorganic fibers with a matrix phase comprised of a ceramic materialdescribed hereinbefore. Combining the coated inorganic fibers with amatrix phase causes the coated fibers to be disposed within andthroughout the matrix phase. Finally, the ceramic matrix composite isthen heated to a temperature sufficient to convert the clay coating onthe fibers to a coating containing mullite. The temperature ispreferably at least about 800° C., and usually no higher than about1700° C. The result is a ceramic matrix composite article wherein theinorganic fibers have a mullite-containing coating which acts as aninterfacial layer between the fibers and the matrix, and thus serves asa debonding interface.

Prior to coating, the suspension can be optionally treated to removeexcess salts. This is preferably done by dialyzing the suspension. Thestep of dialyzing typically comprises placing the suspension in adialysis membrane (which can be purchased commercially from e.g.,Spectrum Medical Industries, Inc. of Los Angeles, Calif.), and placingthe membrane containing the suspension in excess dionized water, e.g.,2000 mL of deionized water for 100 mL of the suspension. The membraneshould remain in the deionized water for a sufficient time to allowremoval of the excess salts. The water is preferably stirredperiodically and replaced after 24 hours.

Additionally, the optional step of removing excess salts can also beaccomplished by filtering the suspension and then washing the collectedfiltrate with water.

Another embodiment of this invention involves a method for coatingfibers with a mullite-containing coating. The method is similar to themethod of forming a ceramic matrix composite article discussed above,but does not include the step of combining the coated fibers with amatrix phase. Instead, the coated fibers can be made and transported forlater use in a ceramic matrix composite. After coating and drying thefibers, the fibers are heated to a temperature sufficient to convert theclay coating to a coating containing mullite, preferably at least about800° C.

Fibers having a mullite-containing coating have not been found to failcatastrophically in a ceramic matrix, and the stress does not fall tozero in four-point bend tests. The failure occurs in steps, gradually,and the ceramic remains in tact. This is a significant improvement overnon-coated fibers which fail catastrophically and the stress goes tozero in the same tests.

EXAMPLES

The following non-limiting inventive examples and comparative examplesare presented to further illustrate the invention.

Comparative Example A Fibers Coated With Bentonite Clay

Silicon carbide fibers, nominally 100 microns in diameter with atungsten core (SCS fiber, Textron Specialty Materials, Lowell, Mass.),were coated with a naturally occurring bentonite clay (Volclay MBS-1,American Colloid Company) by drawing the fibers through an aqueoussuspension of the clay. The fibers, as received, have a polyvinylalcohol coating which was removed by feeding the fiber continuouslythrough a furnace in air at 500° C. The fibers were then coated with thebentonite by feeding them directly from the furnace through a 1.5% (byweight) suspension of the clay. The fibers were dried at 200° C. andre-coated three times. Each dip gave a coating of about 0.03 microns,resulting in a final coating thickness of about 0.1 microns. FIG. 2(a)shows a micrograph from an Scanning Electron Microscope (SEM) of the SiCfiber with a bentonite clay coating showing the uniformity of thecoating. The fibers were then subjected to temperatures up to 1200° C.

Samples of the bentonite clay were taken for x-ray diffraction afterheating to temperatures of 500° C. (FIG. 1(a)-(b)), 800° C. (FIG.1(a)-(c)), 1000° C. (FIG. 1(a)-(d)), and 1200° C. (FIG. 1(a)-(e)).

Example 1 Fibers Coated With Alumina Pillared Bentonite

Mullite-containing debondable coatings were prepared as follows: To astirred aluminum trichloride (2.4 g of AlCl₃.6H₂ O in 50 mL of water)solution, sodium hydroxide (40 mL of a 0.5 M NaOH solution) was addedover about 1 hour to provide a final ratio of OH/Al of 2.0. The solutionwas stirred and heated to 60° C. for two hours. To this solution, 50 mLof a 2% by weight suspension of naturally occurring bentonite clay(Volclay MPS-1, American Colloid Company of Belle Fourche, S. Dak.) wasadded and the mixture was stirred for 24 hours. The resulting suspensionwas then dialyzed (Spectra/Por 3, Spectrum Medical Industries, Inc.) for24 hours with de-ionized water to remove the excess salt formed. Thissuspension of a polyoxo-aluminum Keggin ion [Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺-exchanged bentonite (referred to as Al-bentonite) was used to coatfibers directly. For mechanical testing, silicon carbide fibers,nominally 100 microns in diameter with a tungsten core (SCS fiber,Textron Specialty Materials of Lowell, Mass.), were coated directly withthe clay suspension. The fibers, as received, have a polyvinyl alcoholcoating which was removed by feeding the fiber continuously through afurnace in air at 500° C. The fibers were then coated with theAl-bentonite by feeding them directly from the furnace through thesuspension of the clay. The fiber was dried at 200° C. and re-coatedthree times. Each dip gave a coating of about 0.03 microns, resulting ina uniform coating (see FIG. 2(c)) having a final coating thickness ofabout 0.1 microns. The fibers were then subjected to temperatures up to1200° C.

Samples of the alumina pillared bentonite were taken for x-raydiffraction after heating to temperatures of 500° C. (FIG. 1(b)-(g)),800° C. (FIG. 1(b)-(h)), 1000° C. (FIG. 1(b)-(i)), and 1200° C. (FIG.1(b)-(j)).

X-ray Diffraction Results

The distinction to be made between coating made of bentonite clay(COMPARATIVE EXAMPLE A) and those made of alumina-pillared bentoniteclay (EXAMPLE 1) is revealed by powder x-ray diffraction as shown inFIGS. 1(a) and 1(b). The room-temperature x-ray diffraction pattern ofbentonite clay (FIG. 1(a)-(a)) reveals a characteristic layer spacing of1.27 nm. The presence of quartz and cristobalite impurities is alsonoted and acts conveniently as an internal standard. Upon heating thebentonite clay to 800° C. (FIGS. 1(a)-(b) and 1(a)-(c)), the basicstructure of the aluminosilicate layers is maintained while the layerspacing decreases with loss of interlayer water to 0.96 nm. At 1000° C.(FIG. 1(a)-(d)) the layer structure collapses as evidenced by loss ofthe (100) peak and cristobalite forms (note the intensity change of thecristobalite peak relative to quartz). Presumably, the aluminumoxide-containing component of the clay is amorphous. Upon furtherheating to 1200° C. (FIG. 1(a)-(e)), mullite Al₆ Si₂ O₁₃ is formed andthe excess silica which would be expected for a 1:2::Al:Si bentoniteclay is converted completely to cristobalite.

In FIG. 1(a)-(f) the pattern of bentonite clay exchanged with [Al₁₃ O₄(OH)₂₄ (H₂ O)₁₂ ]⁷⁺ ions is shown. The increase in the layer spacing to1.74 nm indicates that the Keggin ions are intercalated between the claylayers. The rest of the spectrum is similar to that of bentonite (FIG.1(a)-(a)) with the invariant peaks presumably indexing as (hk0). Uponheating Al-bentonite to 500° C. (FIG. 1(b)-(g)), the layer peakdisappears indicating a disorder in the stacking upon formation ofalumina pillars between the sheets. However, the crystallinity withinthe Si-Al-Si oxide sheet is maintained, as evidenced by the continuedpresence of the (100) reflection. At 800° C., Al-bentonite forms mulliteas shown in FIG. 1(b)-(h). As noted above, upon similar heat treatmentof the unexchanged bentonite no evidence of mullite formation wasobserved (FIG. 1(a)-(c)). While this invention is not bound by anyparticular theory or observation, it is speculated that this observationis attributable to the intimate mixing at the atomic scale of the [Al₁₃O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺ Keggin ions with the Si-rich aluminosilicate claylayers. At higher temperatures (1000° and 1200° C.; FIGS. 1(b)-(i) and1(b)-(j), respectively) the mullite pattern sharpens indicative ofincreasing crystallinity and, as in the pure bentonite case, excesssilica converts to cristobalite. The increase in the amount of mulliteformed compared to that in the case of pure bentonite (FIGS. 1(a)-(e)and 1(b)-(j)) is consistent with the increased aluminum content of theAl-bentonite sample.

Example 2 Composite Frabrication Using Coated Fibers

Composites of the coated fibers and a borosilicate glass (Corning Glass7740 of Corning, N.Y.) were fabricated. Several samples with widelydispersed fibers were fabricated to be sectioned and prepared forindentation experiments. Other samples contained approximately 30%fibers (by volume); these were used for four-point bend tests. Thecomposites were laid-up by hand in a 5 cm diameter graphite die(typically 14.0 g of glass to 3.0 g of SiC fiber). These were vacuumhot-pressed in 10 torr pressure at 900° C. for 15 minutes, cooled at 10°C./minute to 570° C., held at 570° C. for 30 minutes, and furnace-cooledto room temperature. A pressure of 13 Mpa was applied duringhot-pressing at 900° C., and released during cooling. The hot-pressedsamples were ejected from the graphite die at room temperature. Sectionsnormal to the orientation of the fibers were cut and polished to a gritsize of 1 micron for the indentation experiments. Samples for bend testswere cut, typically with dimensions of 45 mm×4 mm×1.5 mm.

Comparative Example 3 Composite Fabrication Using Uncoated Fibers

Composites of uncoated fibers and a borosilicate glass (Corning Glass7740) were fabricated. Several samples with widely dispersed fibers werefabricated to be sectioned and prepared for indentation experiments.Other samples contained approximately 30% fibers (by volume); these wereused for four-point bend tests. The composites were laid-up by hand in a5 cm diameter graphite die (typically 14.0 g of glass to 3.0 g of SiCfiber). These were vacuum hot-pressed in 10 torr pressure at 900° C. for15 minutes, cooled at 10° C./minute to 570° C., held at 570° C. for 30minutes, and furnace-cooled to room temperature. A pressure of 13 MPawas applied during hot-pressing at 900° C., and released during cooling.The hot-pressed samples were ejected from the graphite die at roomtemperature. Sections normal to the orientation of the fibers were cutand polished to a grit size of 1 micron for indentation experiments.Samples for bend tests were cut, typically with dimensions of 45 mm×4mm×1.5 mm.

For both coated and uncoated fibers, individual fibers were indentedwith a Vicker's diamond tip (Zwick 3212) with loads in the range of 5-30N. The indentation caused the fibers to debond. The residualdisplacement after indentation was seen to be an order of magnitudegreater for the coated fibers compared to the uncoated fibers. FIGS.3(a) and 3(b) show SEM micrographs of coated and uncoated fibers afterindentation. In all cases, the indentation spawned radial cracks. These,however, do not seriously affect the primary measurement from thisexperiment: the residual out-of-plane displacement of the fiber. Uponunloading, the residual displacement of the fiber with respect to thematrix, u^(r), was used to estimate the frictional resistance of theinterface. Its average value was 0.1±0.02 microns for the uncoatedfibers and 1.0±0.3 microns for both the bentonite and Al-bentonitecoated fibers. Using equation (1) (below), the sliding resistance t wasestimated to be about 1.0 GPa for the uncoated fibers and 100 MPa forthe coated fibers. The sliding resistance for the coated fibers is largecompared to commonly measured values for carbon interfaces, butsufficiently low compared to the uncoated fibers to promote debonding atthe interface in the bend tests described below.

The samples had a relatively small volume fraction of fibers (about30%), and a glassy matrix was chosen for ease of processing, rather thanas a candidate for ultimate use. It is contemplated that performancecould be improved by increasing the volume fraction of the fibers,and/or by using different matrix materials.

The residual displacement was measured directly from scanning electronmicrographs taken at known tilts, and by surface profile measurements.It is used to estimate the (assumed) constant shear t which resistssliding of the fiber-matrix interface. This allows a comparison ofdifferent coatings using a single parameter and a simple indentationtest. Then, t can be estimated as:

    t=F.sup.2 /(8 π.sup.2 R.sup.3 u.sup.r E.sup.f)          (1)

where F is the indentation load, R is the fiber radius, and E^(f) is theYoung's modulus of the fiber (=400 GPa). Four-point bend tests wereconducted in air or in water using inner/outer spans of 10/40 or 20/40mm, at a ram displacement rate of 25 mm/s.

FIG. 4 shows typical results from four-point bend tests for four typesof specimens: (1) glass matrix material; (2) glass matrix with uncoatedfibers; (3) Al-bentonite coated, which is substantially mullite coated,fibers in glass matrix tested in air; and (4) Al-bentonite coated fibersin glass matrix tested in water. The measured force-displacement datawere converted into stress and strain, based on a nominally undamagedmaterial. The three composite specimens have a greater initial stiffnessthan the glass specimen due to the greater stiffness of the fibers. Theglass and the composite without a coating both show brittle behavior.Failure for these specimens was from a single crack with little pull-outof fibers in the composite. The stress-strain behavior of thebentonite-coated composites (data not shown here) is similar to theuncoated composite samples.

In contrast, the composite with the Al-bentonite coating, tested in air,shows distinct non-linearity prior to the peak stress. The test wasinterrupted for a few specimens after the onset of non-linearity butprior to the peak stress. These specimens showed multiple-matrixcracking on the tensile side of the specimen with typical crack spacingof 0.75-1.00 mm. Most of the Al-bentonite coated samples showed a singlelarge decrease in the stress, corresponding to fiber-failure in one ofthe matrix cracks. However, the specimens continued to exhibit stabledeformation by steady-state propagation of a delamination crack alongthe beam. When tested in water, the specimens with the Al-bentonitecoating displayed much greater matrix cracking, and stable deformationwithout fiber failure up to much larger strains. The glass, uncoated,and bentonite-coated specimens continued to fail in a manner similar tofailure in air. FIG. 5 shows the tensile surface of a sample with anAl-bentonite coating after bending in water with multiple matrix cracks.

FIG. 2(b) shows an SEM micrograph of a cross section of a composite ofthe SiC fibers (with tungsten cores) in a glass matrix. FIG. 2(c) showsan SEM micrograph of a SiC fiber with an interfacial mullite-containingcoating in a glass matrix. FIG. 2(c) clearly shows the uniformity of thecoating.

Although particular embodiments of the present invention have beendescribed in the foregoing description, it will be understood by thoseskilled in the art that the invention is capable of numerousmodifications, substitutions and rearrangements without departing fromthe spirit or essential attributes of the invention. Reference should bemade to the appended claims, rather than to the foregoing specification,as indicating the scope of the invention.

What is claimed is:
 1. An improved ceramic matrix composite articlecomprising:(a) a matrix phase comprised of a ceramic material selectedfrom the group consisting of crystalline ceramics, glass ceramics,glasses and combinations thereof; and (b) a fiber reinforcement phasecomprised of a plurality of amorphous or crystalline inorganic fibersdisposed within the matrix phase,wherein the improvement comprises theinorganic fibers having a mullite-containing coating on the surface ofsaid inorganic fibers and wherein the inorganic fibers are not comprisedof a mullite-precursor.
 2. The article of claim 1 wherein the inorganicfibers are selected from the group consisting of silicon carbide,silicon oxycarbide, carbon, alumina, boron carbide, boron nitride,zircon, seine, silicon nitride, silicon oxynitride, titanium carbide,and titanium diboride fibers.
 3. The article of claim 1 wherein thematrix phase is selected from the group consisting of borosilicateglasses, aluminosilicate glasses, lithium aluminosilicate glasses,alkaline earth aluminosilicate glasses, silicon carbide, boron nitride,silicon oxynitride, and silicon nitride.
 4. The article of claim 1,wherein the fiber reinforcement phase comprises at about 30% to 80% byvolume of the ceramic matrix composite article.
 5. The article of claim1 wherein the inorganic fibers are silicon carbide fibers and the matrixphase is silicon carbide.
 6. A process for making a ceramic matrixcomposite article comprising the steps of:(a) modifying a smectite clayby ion-exchange in solution to provide sufficient amounts of aluminumions in the clay; (b) adding a pillared smectite clay to the solution toform a suspension; (c) drawing an inorganic fiber through the suspensionof step (b) and drying the fiber thereafter; (d) repeating step (c) aplurality of times until the desired amount of coating is deposited onthe surface of the fiber; and (e) combining a plurality of coated fiberswith a matrix phase comprised of a ceramic material selected from thegroup consisting of crystalline ceramics, glass-ceramics, glasses andcombinations thereof, to form a ceramic matrix composite such that theplurality of coated fibers are disposed within the matrix phase; and (f)heating the ceramic matrix composite to a temperature sufficient toconvert the clay coating on the fibers to a coating containing mulliteand wherein the inorganic fibers are not a mullite-precursor.
 7. Theprocess of claim 6 wherein the heating temperature of step (f) is atleast about 800° C.
 8. The process of claim 6 wherein the solublealuminum ions are [Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺.
 9. The process of claim6 wherein the soluble aluminum ion is Al⁺³.
 10. The process of claim 6wherein the pillared smectite clay is montmorillonite.
 11. The processof claim 6 further comprising removing excess salts after step (b) andbefore step (c).
 12. The process of claim 11 wherein excess salts areremoved by dialyzing the suspension of step (b).
 13. The process ofclaim 11 wherein excess salts are removed by filtering and washing thesuspension of step (b).
 14. A method for coating a fiber with amullite-containing coating, comprising the steps of:(a) modifying asmectite clay by ion-exchange in solution to provide sufficient amountsof aluminum ions in the clay; (b) adding a pillared smectite clay to thesolution to form a suspension; (c) drawing an inorganic fiber throughthe suspension of step (b) and drying the fiber thereafter; (d)repeating step (c) a plurality of times until the desired amount ofcoating is deposited on the surface of the fiber; and (e) heating thecoated fiber to a temperature sufficient to convert the clay coating toa coating containing mullite and wherein the inorganic fiber is not amullite-precursor.
 15. The process of claim 14 wherein the heatingtemperature of step (e) is at least about 800° C.
 16. The process ofclaim 14 wherein the soluble aluminum ions are [Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂]⁷⁺.
 17. The process of claim 14 wherein the soluble aluminum ion isAl⁺³.
 18. The process of claim 14 wherein the pillared smectite clay ismontmorillonite.
 19. The process of claim 14 further comprising removingexcess salts after step (b) and before step (c).
 20. The process ofclaim 19 wherein excess salts are removed by dialyzing the suspension ofstep (b).
 21. The process of claim 19 wherein excess salts are removedby filtering and washing the suspension of step (b).
 22. An inorganicfiber having a mullite-containing coating made by the process of claim14.
 23. An amorphous or crystalline inorganic fiber having amullite-containing coating wherein the inorganic fiber is not amullite-precursor.
 24. The fiber of claim 23 selected from the groupconsisting of silicon carbide, silicon oxycarbide, carbon, alumina,boron carbide, boron nitride, zircon, spinel, silicon nitride, siliconoxynitride, titanium carbide and titanium diboride fiber.